Fastener preloading is generally considered to be the axial force exerted on a joint by the fastener. However, it is an oversimplification to approximate preloading by calculations of simple axial or longitudinal strain. The geometrical imperfections of a threaded fastener cause interference between the threads and the tapped hole of the corresponding member. Incorrect clamping stress is undesirable because it exaggerates these imperfections. Also, torsional strain, caused by surface friction, is introduced to the fastener within the tapped hole, and additional bending strains are introduced by misalignments such as the non-parallelism between the mating surfaces.
In general, frictional force is a force which resists sliding motion. Frictional force, which is proportional to applied load, can be overcome by translational force. However, the frictional coefficient of a surface is independent of both the applied load and the total surface area in sliding contacts. Thus, the total frictional force, as distinguished from the coefficient of friction, between for example, a threaded fastener and a tapped hole, is a function of the total area engaged in sliding contact, and the mechanical strength and composition of the near surface region of the respective components.
It is desirable to reduce the sliding friction between a fastener and a threaded bore because, with friction force reduced, an increased clamping force is available for a given torque value. Under classic tribological theory, low sliding friction is accomplished by mating a compliant member with another, non-compliant member. For example, the superficial hardness of a carbide pin makes it non-compliant relative to a chrome plated disc upon which it slides. Unfortunately, it is not always practical to provide a matching tribological couple such as a carbide pin and chrome plated disc. In fact, tribological mismatches in various technologies are not uncommon. Of particular significance here is the tribological couple mismatch between an austenitic stainless steel fastener and an aluminum, threaded bore.
Austenitic stainless steel fasteners are commonly used with aluminum. The friction emerging from torquing a stainless steel fastener, which is tribologically mismatched with the aluminum, into the aluminum produces thread galling. The galling which occurs when torquing a fastener into or out of a threaded bore may also be accelerated or exaggerated by the localized heating which results from torquing. The mechanism that produces the thread galling is explained by the "adhesion theory" which teaches that when two surfaces are in sliding contacts, the high points or tips of the opposing surface asperities engage and form metallic junctions due to their elasticity. These junctions result in abrasive deformations and increases in total frictional force. A translational force is required to shear these metallic junctions.
In addition to being tribologically mismatched with stainless steel, aluminum is generally known to be intrinsically abrasive. Aluminum machining or cutting equipment has a notoriously short service life. In various applications, such as avionics, aluminum components are typically protected by a low friction coating to prevent severe wear. Similarly, austenitic stainless steel is an intrinsically poor triboelement. The microstructure of austenitic stainless steel is known to weaken under the vibrational effects of sliding contact. Also, it is generally accepted that the abrasion rate of austenitic stainless steel is moderate to rapid.
Various attempts have been made to reduce sliding friction using solid lubricants. These attempts have generally involved surface processing to trap solid lubricant crystals into the near surface microstructure of the triboelement. For example, U.S. Pat. No. 4,240,886, U.S. Pat. No. 4,312,900, and U.S. Pat. No. 4,528,079 teach attempts to make lamellar solid low friction species surface or near surface components of a triboelement's microstructure to lower sliding friction and improve wear performance. U.S. Pat. No. 4,125,637 teaches a method of impinging the inside diameter of a wet cylinder sleeve with silicon carbide particles dispersed in the cooling oil while honing the cylinder wall to produce cross-hatching and improve geometrical precision.
Present day solid lubricant theory generally includes the uniform dispersal of a low friction species in a highly ordered, mechanically strong matrix. Lead for example, which acts as a solid lubricant, has been incorporated into the surface matrix of copper at concentrations beyond eutectic equilibriums using state of the art melting techniques in alloying technology. Similarly, carbon monofluoride has been co-deposited as a protective surface coating on electroless nickel (see U.S. Pat. No. 4,830,889). The carbon monofluoride is uniformly dispersed as a low friction species into the extremely fine grained, electroless nickel which serves as a highly ordered, strong, support matrix.
It is commonly known that the family of lamellar solids which includes carbon monofluoride, as well as tungsten disulfide and molybdenum disulfide, includes solids having crystal structures which result in low friction sliding. The crystal structure of these low friction species typically have weak Van der Waal type bonding in the plane perpendicular to the basal plane. This weak bonding provides low shear on sliding. The molybdenum disulfide crystal is particularly noteworthy because it exhibits anisotrophy on an order of magnitude of 29 to 1. Along the basal plane, the molybdenum disulfide crystal is fairly strong and can withstand the applied force required to impregnate the crystal into the surface of the triboelement. Thus, low sliding friction is possible using molybdenum disulfide, only if the molybdenum disulfide crystal is introduced into the surface of the triboelement at the proper geometric orientation. This degree of anisotrophy (i.e., inequality of physical properties along different axes), and the accompanying benefits, is not present in the carbon monofluoride crystal. Thus, the crystal structure of the carbon monofluoride species permits its use in nearly every conformal application, and its load carrying capacity will not be exceeded. However, this species exhibits very little anisotrophy. Thus, it is anticipated that carbon monofluoride will be combined with either Molybdenum disulfide or tungsten disulfide for mechanical inclusion according to present invention. It should also be noted that graphite could serve as a suitable lamellar solid material. The thermal stability for this entire family of solid lubricants is satisfactory for the types of applications contemplated by this invention. Molybdenum disulfide begins oxidation at 400.degree. C. (752.degree. F.). Tungsten disulfide begins oxidation at 452.degree. C. (846.degree. F.).